This invention relates to radio-frequency (RF) antennae used in Nuclear Magnetic Resonance (NMR) apparatus. More specifically, the antennae are housed in a generally tubular structure and act to irradiate the sample with RF energy and/or receive NMR signals from the sample.
The relationship relating angular precession and the static magnetic field B.sub.0 in NMR is EQU .omega.=.gamma.B.sub.0
the Larmor equation
where .omega. is the precessional angular velocity, B.sub.0 is the static magnetic field (consisting of the applied homogeneous field and other fields) and .gamma. is the gyromagnetic ratio, which is constant for a particular isotope. Thus different isotopes precess at different angular frequencies. The Larmor equation applies to the situation where an ensemble of nuclei possessing nuclear spin are subjected to a strong static magnetic field. A number of possible energy levels are established by the application of B.sub.0 and its interaction with the magnetic moments of the nuclear spins.
If RF energy is applied at the appropriate Larmor frequency, and in a direction orthogonal to B.sub.0, the population in each energy state will change, leading to a net imbalance in the populations of energy levels. After cessation of the RF energy, the system will attempt to return to its state before RF excitation and in doing so will emit RF energy at the Larmor frequency. This is the NMR signal and may be detected by the same RF coil that irradiated the sample or by different receiver antenna. In NMR, the RF antennae are generally tuned to the desired frequencies, provide homogeneous RF excitation/reception of the sample orthogonal to the static magnetic field and are desirably efficient in operation. Of principal interest are the RF magnetic fields generated by the antenna and these are often termed the B.sub.1 fields in NMR.
Of particular interest in NMR is the use of resonant structures (resonators) such as those known in the art from, for example, EP 0 177 855 B 1. These structures generate essentially homogeneous B.sub.1 fields transverse to B.sub.0. In this transverse plane two essentially homogeneous magnetic field modes are generated in orthogonal spatial directions.
In certain NMR experiments, it is advantageous to incline the sample and the resonator at an angle to the static magnetic field, rather than having the axes of the sample, resonator and main magnet co-incident. A particular case is where the angle of inclination is cos.sup.-1 (1/.sqroot.3) degrees and the sample is rotated (spun) around this tilted axis; this has the effect of removing chemical shift anisotropy and averaging out the effect of dipolar couplings in the sample. This is well known in the art of the NMR of solids (see for example "High Resolution NMR in the Solid State", E. O. Stejskal and J. D. Memory, Oxford University Press, 1994) and is also used in liquid state NMR in an attempt to remove inhomogeneous broadening effects.
Prior art probes that contain RF coils tilted from the B.sub.0 axis use coils that generate RF magnetic fields along the coil axis, such as solenoidal wound structures. These coils suffer from a loss in efficiency in generating NMR useful RF magnetic fields when inclined to the static field B.sub.0 therefore, by reciprocity, also suffer a reduction in signal to noise ratio (SNR) of the received NMR signal. At an inclination of cos.sup.-1 (1/.sqroot.3) degrees, the reduction in the magnitude of the NMR useful RF magnetic field is approximately 18% for fixed power input. A further form of NMR experiment requires that the inclination angle be altered during the course of the experiment. Prior art probes require a re-calibration of the 90 degree pulse time (i.e. the rectangular envelope RF pulse duration for maximum signal return in a simple one-pulse experiment) at each angle.
It is an object of this invention to provide an RF resonator that provides a substantially homogeneous RF field transverse to its resonator axis, said resonator axis being inclined away from the axis of the static magnetic field B.sub.0.
It is a preferred object of this invention to provide an inclined resonator that has one RF mode oriented along the resonator's pivotal axis such that the inclination of the resonator to the B.sub.0 field does not affect the useful RF magnetic field generated by the resonator and such that the SNR of the received NMR signal for a fixed rf power input does not change with inclination angle.